Optical storage media are media in which data are stored in an optically readable manner by means of a pickup including a laser and an optical detector, for example a photodetector. The detector is used for detecting reflected light of the laser beam when reading data on the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-ray disc (BD), which allows to store up to about 50 GB on a dual layer disc. For reading and writing of a Blu-ray disc a pickup with a laser wavelength of 405 nm and a numerical aperture of 0.85 is used. On the Blu-ray disc a track pitch of 320 nm and a mark length from 2T to 8T or 9T is implemented, where T is the channel bit length and wherein 2T corresponds with a minimum mark length of 138-160 nm.
The spatial resolution limit of optical instruments as described by the Abbe theory is about λ/2NA, which is 238 nm for a Blu-ray type pickup having a laser wavelength λ=405 nm and a numerical aperture NA=0.85. 238 nm represents here the period of the smallest detectable mark frequency, period which is constituted of a pit and of a land of the same length. λ/2NA is also called the diffraction limit and corresponds with the focus diameter of the laser beam on the optical disc. It corresponds finally to a theoretically smallest detectable mark (pit or land) of 119 nm (in regard to a Blu-ray optic), or related to the wavelength and the numerical aperture, a smallest detectable mark (pit or land) of λ/4NA=119 nm.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-ray disc. This is possible by including a nonlinear layer, which is placed above the data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, and which center reflectivity is dependent on the pit structure of the corresponding data layer. Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the diffraction limit of λ/4NA of a corresponding optical pickup.
The nonlinear layer is often called a super-resolution near-field structure (Super-RENS) layer because it is assumed that for some specific materials, the optical effect of reducing the effective spot size of the laser beam is based on a near-field interaction between the marks and spaces of the data layer and the nonlinear layer. Super-RENS optical discs comprising a super resolution near-field structure formed of a metal oxide, a polymer compound or a phase change layer comprising a GeSbTe or a AgInSbTe are known.
In WO03034412 an optical storage medium is disclosed including a data layer having substantially parallel tracks being arranged in groups, wherein each group includes at least a first track having broad marks and at least a second track having narrow marks of a second width smaller than the first width to provide a reduced track pitch. A similar storage medium is disclosed in JP-A-05-182203, which has a track structure with tracks being arranged as two nested spirals.
IN WO 2008/071653, an optical storage medium is described comprising a data layer with a mark/space structure arranged in tracks, wherein a sequence of marks of a first track have a first width, and a sequence of marks of an adjacent track have a second width being different from the first width. The tracks are arranged in particular as one or two spirals, circular rings or segmented circular rings on the storage medium.